Add support for gzipped responses

[MarkDavidson]

Like the title says, libtaxii currently can't handle gzipped responses (either Content-Encoding or Transfer-Encoding). This hasn't come up, because libtaxii doesn't specify gzip in the Accept-Encoding header.

Adding this feature will allow library users to specify gzip in the headers and not have to worry about getting an unparseable response back.

-Mark

[gtback]

As we get more into the weeds with various HTTP features, we should seriously consider using requests.

[bworrell]

Yay requests 👍

[MarkDavidson]

👍 for requests as well. It's really about development time to re architect clients.py, and whether we want to do a libtaxii 2.0 (I think there's other backward incompatible changes I'd like to make as well).